The Annelida include the two-segmented worms, earthworms, and ringed
worms.Arrangement of the various groups on Annelida is in a constant state
of flux as specialists continue to disagree on where to place the
members.As of July 2010 the classification
show here tends to have the majority of adherents.

They are the first phylum with
segmentation.Their body is divided
into a linear series of metameres.They have bilateral symmetry; a true coelom and one pair of nephridia
are located in most segments.Nephridiareach their highest point of development in this group.They possess a non-chitinous cuticle but
they have chitinous setae.

The nervous system consists of a
dorsal brain, a ventral, double and solid nerve cord and a ganglion in every
segment

Circulation is via a
well-developed blood system, which is usually closed, and a dorsal blood
vessel or heart.

Most species occur in the marine
environment, but some are terrestrial and others are found in freshwater.Their size varies from microscopic to over
four meters in length. Marine forms have trochophore larvae and thus there is
a common ancestry with the Mollusca.

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The Class Polychaeta,
meaning "many bristles" includes those species that possess tufts
of chitinous setae on every segment of the body.All characteristics of the phylum are present.There is a well-developed head with
sensory structures, such as tentacles, palps and eyes.There are lateral outgrowths called parapodia, which occur on every segment and are equipped with setae.Transient gonadsappear only during the breeding season and occur in many segments
of the body.There are no definite
openings for gametes to exit the body, so they escape via temporary openings
or by rupturing of the body wall.Most are marine organisms, but some do occur in fresh and brackish
water.

An example Genus is Nereis(= Neanthes).The species occur on
coastal shores where they live in tubes in mud or sand, which are lined by a
mucous secretion.

Body Plan. -- The species are long and cylindrical with a series of
segments that are more or less identical except for the posterior and
anterior segments, which have homomous metamerism.There is an iridescent
cuticle, which is caused by fine striations.The mouth is anterior and the anus posterior.The head consists of two parts: (1)prostomiumthat bears palps, tentacles and eyes and (2) peristomiumthat bears cirri and a mouth.The posterior segment bears the anus and anal cirri.

Body Wall. -- There is a cuticle, which is permeated with small holes through
which mucous pores.There is a
glandular epidermis, circular muscles, feathery longitudinal muscles, a
peritoneum and a coelom.The coelom
is divided into partitions, or septa, which correspond to each segment.Pores through the septa allow for inter-communication.

Food &
Digestion. -- All species are primarily predaceous, their food consisting
of small crustaceans and other worms.The food is captured by means of an eversible pharynxthat bears powerful jaws on the everted tip.

There is a short esophagus and a
long intestine where digestion and absorption occurs.The wall of the intestine is well
developed with circular and longitudinal muscles and it is covered by a
peritoneum.

The ventral
blood vesselcollects blood from the
segments and carries it posteriorly.It is usually not contractile.

Commissures connect the two
vessels at the anterior and posterior ends.The blood is found in capillariesor vessels, but there are no sinuses.The blood is dissolved in the plasma or haemoglobin.

Respiration. -- The whole body surface functions in respiration, but the
greatest respiration occurs in the parapodia.The oxygen carrying capacity of the blood is high due to the
haemoglobin.

Excretion. -- One pair of nephridia occurs in all segments except the last
few and the first segments.The
nephridium is composed of a funnel, a tubule and nephridiopore.The funnel portion lies in one segment
while the tubule and pore lie in the adjacent segment.The funnel is called the nephrostome.The tubule is well
supplied with blood vessels and both secretions and excretions pass through
its walls.

Movement. -- There is a worm-like movement, which includes crawling,
burrowing and swimming.Nereis is primarily burrowing.

SenseOrgans. -- The eyes include a lens, pupil and retina.Tentacles, palps, cirri, ventral portions
of the parapodia and the general body surface all are sensory.

NervousSystem. -- Included here are a dorsal brain or cerebral ganglia, a
ventral nerve cord, ganglia in each segment and circumpharyngeal
connectives.

Reproduction. -- Nereis has
transient gonads, which are found in posterior parts of the body and seem to
develop from the peritoneum.There
are no genital ducts and the gametes are shed through temporary openings in
the body wall.Fertilization occurs
in the open water; however, males are attracted to the female while she is
shedding the eggs.

The mating season is predictable,
being regulated according to phases of the moon.In the South Seas a worm, Palololiving in coral burrows has posterior segments bearing gametes
that break off and indulge in mating only.

The SubClass Oligochaeta, meaning "few-bristled" have members with no well
developed head.There are few
bristles such as in the earthworm where there are only four pairs per
segment.Permanent gonads are located
in definite segments of the body.These are hermaphroditic animals with no parapodia.They occur primarily in freshwater and
terrestrial environments, but there are a few marine species.

Body Plan. -- The head is absent and there are no parapodia.Setae are few in number, with four pairs
on each segment:two ventrally and
two ventro-laterally.A swollen
structure, the Clitellum is present
on the epidermis, which secretes a cocoon in which the eggs are deposited.Permanent genital openings occur for both sexes on the same
animal.The male opening is on
segment 15 and the female opening on segment 14.There are also dorsal pores on each segment that open from the
coelom to the outside of the body, but their function is not completely
understood.The last segment bears
the anus.

Body Wall. -- This is similar to the Polychaeta except that the epidermis
is very glandular and secretes mucous from one-celled gland cells.The longitudinal muscle bundles are
distinct, however.

Food &
Digestion. -- Food is obtained from organic
debris and the earth generally.The pharynx
is highly muscular and sucks food into the body.The esophagus is equipped with calciferous
glands, which neutralize highly acid
soils.An enlarged part of the
esophagus is the crop, which is thin-walled and
used for storage.A thick-walled gizzardis present that grinds up food material.

There is an invagination of the
intestine called the typhlosole, which increases the surface area for absorption and secretion
of enzymes.

Circulation. -- This is similar to Nereis
except that aortic loopsthat encircle the esophagus
are present, which connect dorsal and ventral vessels.These loops are not the main pumping
organs but they are the principal mechanism to maintain blood pressure.The blood is made up of haemoglobin in
plasma.

Respiration. -- There is diffusion through the entire body surface, and
there are many capillaries in the epidermis.Hemoglobin in the blood also carries oxygen.

Excretion. -- Paired nephridia are present.Chloragogue tissueaccumulates wastes.This
is situated around the intestine and typhlosole.Pieces of the cells break off into the coelom and are picked up
by nephridia.

Locomotion. -- Contractions of the body and setae that enhance traction
allow the animal to move about.

Nervous System. -- There are no visible sense organs, but sensory cells occur
that are light and taste sensitive.Light
sensitive areas are situated in the anterior and posterior regions, which
taste or gustatory-sensitive areas are anterior.

Reproduction. -- Lumbricus is
hermaphroditic and only cross-fertilization occurs where each animal acts
each sex during copulation.

The organs involved in
reproduction are the testes, of which there are two pair on segments 10 and
11.Seminal vesicles enclose the
testes and are used for storage and maturation of sperm.There are three pair joined at the base.The vas deferens joins to two pores.Male genital pores are on the 15th
segment.One pair of ovaries occur on
the 13th segment under the intestine.The oviduct is connected to a pore on segment 14.Two seminal receptacles are on each of
segments 9 and 10.They open to the
outside by separate ducts and are used to store sperm received from male
organs of other animals.A clitellum
secretes mucous to hold worms together during copulation and also to secrete
a cocoon on segments 33-37.

During copulation each worm
exchanges sperm to the seminal receptacle of the other.The clitellum secretes a cocoon around the
body.As the worm moves the cocoon
begins to move interiorly, picking up eggs as it passes over the female pore
and sperm and albumen as it passes over the seminal receptacle pore.The ends of the cocoon are then closed.

Fertilization is external in the
cocoon.Development in the cocoon
involves no larval stage and a miniature worms hatch out.

Asexual reproduction occurs which
involves budding.

Economic Importance. -- Earthworms are valuable in their ability to aerate and
fertilize soils.

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Please see
following plates for Example Structures of the Oligochaeta:

In Polychaeta a parapodium is present,
the sexes are separate and gonads are temporary.There is a head with distinct sensory organs.There is a free-swimming trochophore larva
and all members occur in marine environments.Asexual reproduction is by budding.Haemoglobin is present.Some species have a green pigment with iron called chlorocruorin.

The Oligochaeta do not have a
parapodium.They are hermaphroditic
with permanent gonads.The head is
indistinct and there are no sense organs.Development is direct with no larval stage.They inhabit terrestrial and freshwater environments.There is also a budding asexual
stage.Haemoglobin is present and
Chloragogue tissue accumulates wastes.

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The SubClass Branchiobdellahas only about 150 species of small animals that are
mainly parasites or commensals on crayfish),

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The SubClass Hirudiniaincludes the leeches, which
are blood-sucking parasites of other animals.They are primarily freshwater animals, but terrestrial and
marine members exist.Their
characteristics bear a closer resemblance to Oligochaeta than Polychaeta.

Body Plan. -- There is a higher degree of flattening than in the other
members of the Annelida.They possess
conspicuous suckers on both ends of the body,
and there are a definite number of segments for each species in the adult
form.Segments are subdivided into
"annuli" which are a secondary segmentation. A cluster of simple eyes is situated on each side of the head (Inv114).

Food &
Digestion. -- There are some predatory species
but most are parasitic on other organisms.They have three chitinous jaws that are able to make incisions.Salivary glands prevent coagulation of a
host's blood.The pharynx sucks the
host's blood into the body.The crop
is very large with many lateral pouches that store food.There is a stomach, intestine, rectum and
anus.

Circulation. -- There is a modified circulation where the blood goes into
sinuses, which are remnants of the true coelom.These are lined with gelatinous tissue called botryoidal
tissue.This reduced and modified coelom differs from all the other
Annelida.Part of the coelom is used
as a haemocoel, and is thus thought of as a partially open system.

Respiration. -- Capillaries are present in the epidermis and oxygen diffuses
through the skin.

Motion &
Locomotion. -- There is swimming and a
measuring worm movement that is facilitated by suckers.

Nervous System. -- Sense organs are the eyes and papillae on the anterior
end.Nerves are present as in other
Annelida.

Reproduction. -- This is similar to the Oligochaeta as they are
hermaphroditic and cross-fertilization occurs.The clitellum is not visible externally as there is no
swelling.Development is direct.In some species the sperm are deposited on
the body and they make their way through the body wall and into the cavity.

Economic Importance. -- Hirudinia have been implicated in the transmission of some
diseases.Their attachment to
swimmers in lakes causes fright and minor irritation.

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The Class Myzostomidaincludes a group of small parasitic worms
that live on crinoids, a type of echinoderm. They were first discovered in
1827. Some species, as Myzostoma cirriferum, move around on the
host.Myzostoma glabrum
remains motionless with its pharynx inserted in the mouth of
the crinoid. Myzostoma deformator produces a gall on the arm of
the host, one joint of the pinnule growing around the worm enclosing it in a
cyst while Myzostoma pulvinar lives in the alimentary canal of
a species of Antedon

A typical animal has a flattened rounded shape, with a thin edge drawn
out into tiny radiating hairs or cirri. The dorsal surface is smooth, with
five pairs of parapodia on the bottom.Parapodia are armed with hooked setae, by means of which the worm
adheres to its host. Past the parapodia are four pairs of organs, called
suckers. These organs are believed to be sensoryand are comparable to the lateral sense organs of Capitellids.
The mouth and cloacal opening are usually at opposite ends of the bottom
surface. The former leads to a protrusible pharynx, from which the oesophagus
opens into an intestinal chamber with branching lateral diverticula. There is
no observed vascular system. The nervous system consists of a
circumoesophageal nerve, with a slightly differentiated brain, joining below
a large mass of ganglia. The dorsoventral and the parapodial muscles are
developed, but the coelom is reduced mostly to branched spaces in which the
genitalia mature.

Full-grown myzostomids are hermaphroditic. Their internal organs
consist of a branched sac that opens to the exterior or each side of the
animal. Paired ovaries discharge eggs into a median chamber with side
branches, referred to as the uterus, from which the ripe ova are discharged
by a mediar dorsal pore into the end of the rectum.

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The Class Arachianelida (Haplodrili)is sometimes considered as
an order in the Polychaeta.It is a
small group of marine animals that do resemble the Polychaeta.However, they are most primitive in some
ways and quite advanced in others.Segmentation is indistinct externally, but very obvious internally.Parapodia and setae are absent.Nervous tissue is in contact with the epidermis
and there is only one brain ganglion.Nephridia are present.

These animals are hermaphroditic
or dioecious.There is a trochophore
in the developmental cycle.They have
no known direct economic importance, but like the Polychaeta they may serve
as food for other animals (See illustrationsInv115, Inv116,
and Inv117).